1. Field of the Invention
The present invention relates to a method and an apparatus for treating an ammonium-containing liquid, in particularly to a technology for biologically denitrifying ammonium in an ammonium-containing liquid generated in various fields such as the field of wastewater treatment and the field of fine chemicals.
2. Description of the Related Art
Ammonium containing liquids with low to high concentrations are generated in industrial wastewater, photographic development wastewater, and wastewater from chemical factories to produce chemical products, for example. Since those ammonium-containing liquids cause eutrophication and lowering the level of dissolved oxygen in waters if discharged as wastewater, it is necessary that ammonium be purged before discharging the liquids. Furthermore, it is necessary that ammonium be purged from liquid chemical products containing ammonium in order to increase purity of the chemical products.
Conventionally, ammonium-containing liquids with low concentrations are oxidized by chlorine or are treated biologically. In the chlorine treatment, chlorine is reacted with ammonium to remove ammonium, and produces chloramine at the same time. The chloramine may disturb the ecosystem in the environment by a highly bactericidal effect thereof. In addition, the chlorine treatment requires a large amount of chlorine. Therefore, the chlorine treatment is not used for treating ammonium-containing liquids with middle to high concentrations, and generally, such the liquids are treated biologically by nitrification and denitrification.
Such the biological treatment by nitrification and denitrification is performed by nitrification in which ammonium is converted into nitrate through nitrite by nitrification bacteria, and denitrification in which nitrate is converted into nitrogen gas by denitrification bacteria. However, in the treatment of ammonium-containing liquids by nitrification and denitrification, an organic substance is required in denitrification reaction, in other words, it is necessary to add methanol as the organic substance in an amount three times greater than nitrogen. Therefore, since a greater amount of methanol is used as the concentration of ammonia is higher, such the treatment has a disadvantage in that not only an initial cost but also a running cost is increased.
In order to resolve this problem, recently, anaerobic ammonium oxidation is suggested as efficient biological treatment (see Japanese Patent Application Publication No. 2001-037467, for example). The anaerobic ammonium oxidation comprises the steps of: converting a part of ammonium into nitrite by nitrite-type nitrification reaction using ammonium oxidizing bacteria; and denitrifying the nitrite and the remaining ammonium using anaerobic ammonium oxidizing bacteria. The anaerobic ammonium oxidation requires only a small amount of oxygen in the nitrification, and does not require an organic substance in the denitrification. Therefore, it is possible to reduce a running cost for the treatment, considerably.
Although the treatment of ammonium-containing liquids by anaerobic ammonium oxidation can considerably reduce the running cost as described above, the nitrification for converting a part of ammonium into nitrite is not stable, and thus the concentration of nitrite to be reacted with ammonium tends to be changed over time. Therefore, it is difficult to react to nitrite with ammonium by denitrification constantly at a preferable ratio between nitrite and ammonium, and then ammonium-containing liquids cannot be treated in a stable manner, thereby causing treatment quality of the liquids to vary.